THE dream of propelling spacecraft across the Solar System using 鈥渟ails鈥� that catch streams of photons from the Sun could become a reality, thanks to a new technique that allows the delicate sails to be made in space.
NASA is considering several lightweight spacecraft designs that feature components made of gossamer 鈥� a strong but elastic polymer only a few micrometres thick that can be made into sheets thousands of times lighter than their aluminium equivalent. The original idea was to send folded sheets of the material into orbit, where they would be unfurled to protect spacecraft from space dust or sprayed with a reflective aluminium covering to catch photons and sail out to Pluto and beyond.
But having developed two such polymers, called CP1 and CP2, a couple of years ago, NASA hit a few snags when it came to packing and unpacking the stuff. 鈥淚t鈥檚 prone to static and tears easily,鈥� says Kevin White of Physical Sciences, an aerospace company based in Andover, Massachusetts.
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Now White, John Lennhoff and colleagues at the company believe they have solved these problems. They have adapted a technique called electrospinning that uses electrostatic forces to draw droplets of a polymer solution into very fine filaments.
Their idea is to make solid blocks of the gossamer on the ground, ship them into orbit and turn them into the required sail shapes up there. Once in space, the solid polymer would be forced through a set of hot nozzles, melting it to form fine filament-like jets. These hit a cool surface such as a roller and turn into thin sheets of fibres that overlap to form a sail.
In the lab, the team found that by giving the nozzles a positive charge they could better steer the jets of polymer towards a negative electrode surface. This allows deposited fibres to be hundreds of times thinner than before, White told a meeting of the Fiber Society in Natick, Massachusetts, last month.
The technique needs a clean environment or good vacuum to work properly, otherwise tiny particles of dust also become charged and lodged in the material. So Lennhoff began to wonder whether it might not be easier to carry out the final process in space. Since the polymer could be shipped in solid form, rather than dissolved in solution as happens in conventional manufacture of thin films, the launch weight would be kept to a minimum.
The company is now developing a machine that could scale a mast and boom attached to a spacecraft while overlaying strips of the gossamer (see Graphic). And it should be possible to stick individual sheets together to create composite sheets up to a square kilometre in area, says Lennhoff.
Another advantage with the technique is that its fibres are so thin that they scatter and reflect light. That removes the initial need to coat the sheets in aluminium to turn them into a solar sail, as each photon will exert a force on the reflective surface, imparting a tiny amount of momentum to the sail.
Keith Belvin, who works on solar sails at NASA鈥檚 Langley Research Center in Hampton, Virginia, says the new technology has a lot of potential but he still expects the first solar sails will be made on the ground. 鈥淏ut in-orbit processing is something we would like to do,鈥� he says.